In recent years, wireless power transfer technique has been attracting attention, which technique allows electric power or energy to be transferred without using physical wires or cables. In transferring power of several watts or more to a distance of several centimeter to several tens centimeter, electromagnetic induction or coupling is typically used by providing coils in a power transmitter and a power receiver. Especially, power transfer technique making use of magnetic resonance is highly expected to wirelessly supply electric power to multiple power receivers, or to a receiver that takes three-dimensional dynamic postures. See, for example, Non-Patent Document 1 listed below.
An example of magnetic resonant wireless power transfer system that can deal with three-dimensional dynamic postures is one designed for supplying power to a capsule endoscope inserted in the body of a person being treated. This wireless power transfer system has a detector to collect information about the configuration state of a power receiver, a memory unit to store data of the magnetic field emitted from a power transmitter, and a controller to selectively drive one or more power transmitters via one or more drivers. See, for example, Patent Document 1 listed below.
Another known technique is to produce a rotating magnetic field in an x-y plane using two transmitter coils 101 and 102, as illustrated in FIG. 1. The transmitter coils 101 and 102 are arranged such that the normal vectors of these two coils point in the x direction and the y direction, respectively. By shifting the phases of the oscillating magnetic fields produced by the transmitter coils 101 and 102 from each other by π/2 radians (90 degrees), the resulting synthetic magnetic field becomes the rotating field in the x-y plane. See, for example, Non-Patent Document 2 or Non-Patent Document 3 listed below.